This invention relates to a beamforming method for smart antenna arrays, and in particular relates to a beamforming method for vertically oriented smart antennas arranged in two-dimensional arrays.
Smart antenna arrays, otherwise known as adaptive antenna arrays, comprise of low gain antennas connected to a combining/beamforming network. Smart antennas can provide enhanced coverage through range extension, hole filling and improved building penetration. By improving transmission and reception at the base station, the tolerable path losses can be increased whereby the range of the base station can be improved. By using smart antennas at the base station, initial deployment costs of a wireless network can be reduced: with the development in system usage, system capacity can be increased by adding additional cell sites and decreasing the range of existing base stations.
Digital beamforming, whereby smart antenna arrays are conveniently implemented, is considered as one of the most promising techniques for forthcoming mobile telecommunications networks such as UMTS networks. Linear and planar antenna arrays are normally considered as candidate antenna types, since they produce low sidelobe levels with respect to other types of antenna such as the circular array. If a communications antenna array produces high sidelobes in the beampattern, it will cause strong interfering signals to mobile terminals in some directions, which can be a severe problem for downlink transmission.
In the current UTRAN specification (R""99), there is a transmit diversity scheme based on the feedback from mobile terminals. The idea is to transmit different signals of constant power from two different antennas to all the mobile terminals in a cell. The mobile terminals report the signal magnitude and phase to the base station periodically, upon which the base station adjusts its antenna weights for the transmission of dedicated signals. Currently, within 3GPP, there is a plan to extend the transmit diversity scheme to four antennas, but it is, at present, unlikely that such a technique will ever be extended further to a greater number of antennas.
The present invention seeks to provide an improved antenna arrangement suitable for use in a cellular base station. The present invention further seeks to provide an antenna arrangement operable to increase the capacity of a cellular communications base station.
In accordance with a first aspect of the invention, there is provided a smart antenna basestation arrangement comprising a 2-dimensional array of antennas, which antenna elements provide omni-directional beam coverage in the horizontal plane, wherein:
stored beamforming weight sets optimised for specific beam directions are operable to enable the array to generate directional downlink beams.
Preferably the array has a physical periodicity whereby to reduce the number of principal weight sets for specific beam directions. This periodicity can conveniently comprise a small angular range for a regular circular array of many antennas, a 90xc2x0 angular range for a balanced square array, a 120xc2x0 angular range for a regular triangle and a 180xc2x0 angular range for a rectangle, for example, the specific beam directions being associated with an angular range. In a case where the array comprises a circular array with regular spacing between the elements, for angles between the selected beamforming weights, the weights can be determined by means of rotation and/or interpolation. That is to say, instead of having a single weight set to cover all angles of beam direction about the antenna, there is a weight set for a limited arc, e.g. 30xc2x0xe2x80x94such a weight set can be rotated in 30xc2x0 steps, twelve times to provide 360xc2x0 coverage about the antenna. If the 3 dB beamwidth is less than 20xc2x0, then interpolation is required for angles between the period.
In accordance with a further aspect of the invention, there is provided a base station arrangement operable to exploit the feedback signalling technique proposed in 3GPP. This technique has been developed specifically for downlink beamforming in UTRAN, but they can also be used for uplink beamforming as well as in other wireless systems. A pilot (common control channel) signal may be transmitted by one antenna for a sub-array: feedback signals may be employed to assist the determination of an appropriate weight set.
A first aspect of the invention thus provides an arrangement operable to generate low sidelobe beampatterns for 2-dimensional arrays. A further aspect addresses the feedback signalling technique presently being considered by 3GPP for four antennas in large arrays. The invention is suitable for CDMA wireless cellular systems, as presently determined for third generation cellular wireless networks, and other wireless systems such as TDMA systems and wireless LANs.
A regular two dimensional array of omni-directional antennas and, in particular the circular array, has a number of advantages over linear and planar arrays for the application of smart antennas in wireless cellular networks. If elements with omni-directional patterns in the horizontal plane are used, every antenna element, or at least a high proportion of the antenna elements, will be able to contribute to the transmission and reception of radio signals, thus resulting in the benefits, inter alia, of: low cost of linear power amplifiers (LPA) due to lower power handling requirements; higher antenna gain; higher spatial diversity gain due to antenna distribution, and; signalling for soft (intersector) handover is redundant, which provides a further capacity increase.
The present invention therefore can assist in the realisation of advantages with omni-directional smart antennas based on 2-dimensional arrays. For example, the present invention provides, in one embodiment, a circular antenna array with low sidelobes in the beampattern, thus reducing interference to other users and enabling the use of SDMA; enabling higher downlink capacity; allowing the coexistence of higher data rate services with voice services; and making it possible to apply the 3GPP feedback technique for four antennas to large arrays of vertically oriented omni-directional antennas, the arrays being linear (in two dimensions), circular or planar.